1. Field of the Invention
This invention relates to a hydraulic circuit means for supplying fluid under pressure discharged by a hydraulic pump to a plurality of hydraulic driving means such as hydraulic motors or hydraulic cylinders or the like (which are referred to simply as "hydraulic actuators" hereinbelow).
2. Description of the Related Art
In order to supply fluid under pressure discharged by a hydraulic pump to a plurality of hydraulic actuators, it is necessary to provide a plurality of operating valves in the delivery passage of the hydraulic pump and supply fluid under pressure to each of the hydraulic actuators by operating each of the operating valves. However, if during such a procedure, fluid under pressure is supplied to the plurality of hydraulic actuators at the same time, fluid under pressure is supplied only to hydraulic actuators whose load is low, and is not supplied to hydraulic actuators whose loads are high.
A hydraulic circuit addressing these above-mentioned difficulties, for example, is shown in Japanese Laid-open Patent Application No. SHO 59-197603, has been proposed.
This hydraulic circuit is shown schematically in FIG. 1. In this hydraulic circuit, a hydraulic pump 1 has a fluid discharge passage 2 which is provided with a first operating valve 3 and a second operating valve 4; a first circuit 7 connecting the first operating valve 3 and a first hydraulic actuator 5; and a second circuit 8 connecting the second operating valve 4 and a second hydraulic actuator 6. The first circuit 7 and the second circuit 8 are provided with a first pressure compensating valve 9 and a second pressure compensating valve 10, respectively. The arrangement provides that the load pressure P.sub.1 and P.sub.2 in the first and second hydraulic actuators 5 and 6, respectively, are introduced into a shuttle valve 11, where P.sub.1 is compared with P.sub.2 As a result, a higher load pressure is supplied to the first and second pressure compensating valves 9 and 10, respectively, in order to set the latter at the higher load pressure and keep the pressures at the outlets of the first and second operating valves 3 and 4 equal to each other so that when the first and second operating valves 3 and 4 are operated at the same time the fluid under pressure can be supplied to the first and second hydraulic actuators, respectively, at a flow division ratio which is proportional to the area of openings of the spools of the operating valves.
In such a hydraulic circuit, in order to prevent the load in the first hydraulic actuator 5 at the time of starting from becoming excessive so as to prevent the load pressure at the time of starting from becoming abnormally high, the first circuit 7 is provided with a safety valve 12 so as to relieve the abnormally high load pressure at the time of starting the first hydraulic actuator 5 to thereby set the safety valve 12 at a pressure P.sub.0.
Therefore, when the first and second operating valves 3 and 4 are operated at the same time, the pressure P.sub.0 for setting the safety valve 12 becomes higher than the load pressure P.sub.2 in the second hydraulic actuator 6, and the setting pressure P.sub.0 is applied through the shuttle valve 11 to spring chambers 9a and 10a of the first and second pressure compensating valves 9 and 10, respectively, thereby restricting the area of opening of each of the first and second pressure compensating valves 9 and 10 to set the latter at a pressure corresponding to the setting pressure P.sub.0. As a result, the fluid under pressure discharged by the hydraulic pump 1 is restricted when passing through the second pressure compensating valve 10, and at the same time relieved from the safety valve 12 to the fluid tank, thereby reducing the amount of fluid to be supplied to the second hydraulic actuator 6, thus reducing the operating speed of the second hydraulic actuator 6.
In case, for example, the first hydraulic actuator 5 serves as a turning motor for a power shovel and a second hydraulic actuator 6 serves as a boom actuating cylinder, and a boom is moved upwards by the boom actuating cylinder while the upper turning body is being gyrated, the load pressure at the time of starting the turning motor becomes higher than the pressure P.sub.0 at which the safety valve 12 is set, while the load pressure at the time of starting the boom actuating cylinder becomes lower than the pressure P.sub.0 at which the safety valve 12 is set.
As a result, the area of opening of the second pressure compensating valve 10 is restricted by the pressure P.sub.0 at which the safety valve 12 is set, and fluid pressure is relieved from the safety valve 12 to the fluid tank, and in consequence the fluid under pressure to be supplied to the boom actuating cylinder is reduced, thereby reducing the operating speed of the boom actuating cylinder, which results in reduction in the upwardly moving speed of the boom, thus causing insufficiency in the amount of upward movement thereof.